Effect of lipophilic ion-exchanger leaching on the detection limit of carrier-based ion-selective electrodes.

The equilibrium partitioning of lipophilic ion-exchanger salts from ion-selective polymeric membrane electrodes (ISEs) and its possible effect on the lower detection limit of these sensors is described. Predictions are made on the basis of various parameters, including the knowledge of tetraphenylborate potassium salt partitioning constants, the selectivity of ionophore-free ion-exchanger membranes, and ionophore stability constants in the membrane. Ion-exchanger lipophilicities are significantly increased if the membrane contains an ionophore that strongly binds the primary ion. Predicted detection limits are on the order of 10(-5)-10(-8) M for ionophore-free membranes, and may reach levels as low as 10(-18) M with adequate ionophores in the membrane. Experiments are performed for well-described lead-selective membranes containing different tetraphenylborate derivatives, and detection limits appear to be independent of the ion-exchanger used. However, they are much higher if a more hydrophilic carborane cation-exchanger is incorporated in the membrane. The first finding confirms recent theory, which states that transmembrane ion fluxes, given by a small level of ion-exchange at the sample side by interfering ions, normally dictate the detection limit of these sensing systems. Predicted detection limits on the basis of ion-exchanger leaching alone are here listed for a number of analytically relevant cases. For potassium-selective electrodes containing BME-44 and tetraphenylborate as ion-exchanger, the experimental detection limits are in agreement with predicted values. These results suggest that the detection limit of many current ISEs for ultratrace level analysis are, in optimal cases, dictated by transmembrane ion fluxes; however, because improved chemical solutions are being developed to reduce such effects, simple ion-exchanger partitioning may indeed become an important mechanism that can give higher detection limits than practically desired, and should not be ruled out.

Liquid chromatographic determination of tacrine and its metabolites in rat bile microdialysates.

A microbore liquid chromatographic method using a phenyl stationary phase was developed for the determination of 9-amino-1,2,3,4-tetrahydroacridine (tacrine, THA) and its metabolites in microdialysis samples of bile. Analysis of microdialysis samples requires analytical methods with low detection limits and small sample volume requirements. The method uses a 1-mm I.D. phenyl column and fluorescence detection. A detection limit of 0.25 ng/ml in a 5-microliters sample was achieved for THA. This method was then used to determine THA and THA-1-ol in the bile dialysate of a rat. Because of the small sample volume requirements, a 10-min temporal resolution was achieved for the microdialysis experiment. The low detection limit allowed the THA concentration in the bile to be monitored for more than 4 h following a 1.0 mg/kg i.v. dose of THA.

Simplified dual-lumen catheter design for simultaneous potentiometric monitoring of carbon dioxide and pH.

A novel dual-lumen catheter electrode design suitable for the simultaneous measurement of PCO2 (partial pressure of carbon dioxide) and pH in flowing blood is described. The probe is fabricated from a single segment of dual-lumen silicone rubber or polyurethane tubing that is impregnated with the proton ionophore tridodecylamine. The impregnation step imparts H+ permselectivity to both inner and outer walls of the tubing. By filling each lumen with a suitable buffer/electrolyte solution and Ag/AgCl reference electrode wire, simultaneous potentiometric detection of both PCO2 and pH is achieved. Careful optimization of incorporated proton carrier (tridodecylamine), plasticizer (o-nitrophenyl octyl ether), and lipophilic counteranion sites (tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) within the tubing walls yields catheter electrodes with resistance values of 10-20 M omega and relatively high stability in flowing blood. Results from continuous measurements of PCO2 and pH during long-term 30-65-h blood loop experiments demonstrate that, after an initial conditioning period, the catheter exhibits low drift rates (PCO2, 4.7 +/- 1.7 mV/30 h; pH, 1.4 +/- 0.5 mV/30 h) and yields continuously measured values in good agreement with those obtained on discrete samples with a commercial blood gas analyzer (PCO2, r2 = 0.997; pH, r2 = 0.915). In vivo evaluation of the catheter sensors, performed by implanting silicone rubber dual-lumen probes in the arteries of anesthetized dogs, indicates that the proposed catheter design can closely follow PCO2/pH changes induced in the animals during 6-13 h of continuous monitoring.

Distribution of tacrine across the blood-brain barrier in awake, freely moving rats using in vivo microdialysis sampling.

Microdialysis was used to sample simultaneously the distribution of THA (9-amino-1,2,3,4-tetrahydroacridine; Tacrine), a potential anti-Alzheimer agent, both in blood and across the blood-brain barrier of anesthetized and awake, freely moving rats. Microdialysis probes were implanted in the jugular vein and dorsal hippocampus and dialysis samples were simultaneously collected from both sites. Dialysis samples were analyzed using a microbore column chromatographic assay with a detection limit of 0.3 ng/ml. Pharmacokinetic parameters were calculated after a 1 mg/kg intravenous dose of THA. Plasma pharmacokinetics followed a biexponential mode, with t1/2(dis.) = 8.4 +/- 2.7 min and t1/2(elim.) = 76.7 +/- 24.2 min for awake, freely moving rats. THA rapidly penetrated the blood-brain barrier, with maximum concentrations attained within 60 min post-dose. In the brain of awake, freely moving rats t1/2(abs.) was 26.0 +/- 5.2 min and t1/2(elim.) was 99.1 +/- 17.7 min. THA levels in hippocampus extracellular fluid were 10 times lower than those in plasma. For anesthetized rats, the t1/2(elim.) in blood was 154.8 +/- 46.8 min, while in the hippocampus t1/2(elim.) was 159.5 +/- 31.7 min. The binding of THA in rat plasma was 56.2 +/- 5.0%, while the fraction bound to rat whole blood was 73.3 +/- 4.1% as determined by microdialysis and ultrafiltration.

Intravenous microdialysis sampling in awake, freely-moving rats.

Intravenous microdialysis sampling in the awake, freely-moving rat for the determination of free drug concentrations in blood is described. Intravenous microdialysis was performed with a nonmetallic, flexible dialysis probe. The pharmacokinetics of theophylline were determined using both microdialysis sampling and collection of whole blood following an iv dose. There was no difference in the half-life of elimination of theophylline determined by microdialysis, 4.4 +/- 0.4 h, and whole blood sampling, 4.5 +/- 0.7 h. The kinetics of elimination were affected by removing blood samples and by using anesthesia. The half-life of elimination was 4.4 +/- 0.4 h when using simultaneous microdialysis and whole-blood sampling and only 3.0 +/- 0.4 h using microdialysis alone. The half-life of elimination was 17.0 +/- 7.1 h in chloral hydrate anesthesized rats. Microdialysis samples were continuously collected for over 7 h without fluid loss using a single experimental animal. Microdialysis sampling directly assesses the free drug concentration in blood. The extent of theophylline binding to blood proteins was determined in vitro in rat plasma and rat whole blood using both ultrafiltration and microdialysis. Theophylline was (47.3 +/- 1.3)% bound in rat plasma and (52.2 +/- 1.6)% bound in rat whole blood. Microdialysis sampling is a powerful tool for pharmacokinetic studies, providing accurate and precise pharmacokinetic data.

High-performance liquid chromatographic determination of nifedipine, nicardipine and pindolol using a carbon fibre flow-through amperometric detector.

The electrochemical properties of the calcium-channel blockers, nifedipine and nicardipine, and the beta-blocking agent, pindolol, have been exploited for the determination of their concentrations in plasma samples. High-performance liquid chromatography (HPLC) separation was carried out on a cyanopropyl modified column and the drugs were detected in a flow-through carbon fibre microelectrode cell. The chromatographic system was coupled to a column-switching arrangement in order to perform on-line solid-phase extraction of the drugs from spiked human plasma. Preliminary investigations showed the response of the method to be linear over a range of 20-500 ng ml-1 in plasma with a limit of detection of approximately 15 ng ml-1 for each compound.

Adsorptive stripping voltammetric determination of pipemidic acid in human urine.

The electrochemical behaviour of pipemidic acid (8-ethyl-5,8-dihydro-5-oxo-2-(1-piperazinyl)-pyrido[2,3-d]pyrimidine-6- carboxylic acid), a well known antimicrobial agent used for urinary infections, was investigated by linear-sweep, differential-pulse and square-wave voltammetry at a hanging mercury drop electrode. Two reduction processes were observed in Britton-Robinson buffers at acid pH, whereas only one or two processes were observed in alkaline solutions, dependent on the pH of the buffer employed. Adsorptive effects were used to accumulate the drug on to the electrode. The adsorbed species were measured voltammetrically by using a cathodic process appearing at -0.76 V in 0.1 M HCIO4. Linear calibration graphs were obtained in the range 2.5 x 10(-9)-2.0 x 10(-7) M. A simple procedure of extraction was employed for the determination of the drug in urine samples.

Comparison of a calixarene-based ion-selective electrode with two automated analyzers for the clinical determination of sodium in blood plasma.

Neutral-carrier ion-selective electrodes based on methyl p-t-butylcalix[4]aryl acetate have been prepared that are responsive to sodium ions. The miniaturized catheter-type electrodes were obtained by dip-coating their porcelain tips in a PVC membrane cocktail. Examination of the general performance of the electrodes revealed excellent characteristics in terms of Nernstian response, selectivity, stability, reproducibility and response time. The results from the indirect potentiometric assessment of a large number of plasma samples with the electrodes showed a good correlation with the results from two automated analyzers (Technicon Smac 3, Hitachi 704) and with flame photometric data. Although inconsistencies were observed in the measurement of some plasma samples, the variance seemed to be method-dependent, and the overall performance of the electrodes showed promise as an alternative to the sodium glass electrode. Some factors influencing the standard potential of the measuring cell are discussed as a source of error.